Plex is a huge app and there is no way we would attempt to reproduce their documentation in ClearOS. If for no other reason, it would be impossible to keep it up to date with any changed. It is the same for most third party apps. We tend to document just what is in the webconfig and sometimes take it a bit further (e.g OpenVPN). There is a link at the bottom of the documentation which takes you to Plex's site.

I agree it would help if we said how to get to Plex and I'll add that to the documentation sometime. You should point your browser to http://your_LAN_IP_or_FQDN:32400. You can put your files where you want. I use a custom location. I believe you can use a flexshare as long as you enable it for Third Party Access, and then you have to point Plex to it.

If you fancy helping with the documentation, you are welcome to contribute.

This doesn't really clear much up for me. For example, I installed Plex Media Server, but where/how do I add videos? How do I sign in to my account? I could't find any of that in the documentation...

Try the documentation - https://www.clearos.com/resources/documentation/clearos-7-documentation-overview. Then look at the Use Guides. Each app has its own documentation - the sloping book icon at the top right of the app screen. These docs are the same as the User Guides.